1. Field of the Invention
The present invention relates to a boundary acoustic wave device for use as a resonator or a filter device, for example, and more particularly to a boundary acoustic wave device including a three-medium structure in which a silicon oxide layer and a dielectric layer made of material exhibiting a higher acoustic velocity than that in the silicon oxide are stacked on a piezoelectric substrate made of LiTaO3.
2. Description of the Related Art
Attention is focused on a boundary acoustic wave device instead of a surface acoustic wave device. The boundary acoustic wave device does not require a package having a cavity and hence contributes to a further reduction of size.
WO2008/044411 discloses a boundary acoustic wave device 101 having a sectional structure illustrated in FIG. 10. In the boundary acoustic wave device 101, a groove 102a is formed in an upper surface of a LiNbO3 substrate 102. An IDT electrode 103 is formed by filling a metal material in the groove 102a. In FIG. 10, only one electrode finger of the IDT electrode 103 is illustrated in an enlarged scale.
A silicon oxide film 104 is formed to cover an upper surface of the LiNbO3 substrate 102. WO2008/044411 states that because an upper surface of the IDT electrode 103 is flush with the upper surface of the LiNbO3 substrate 102, an upper surface of the silicon oxide film 104 can be made flat, and hence an insertion loss can be reduced.
Further, WO2008/044411 states that a loss in utilizing an SH boundary acoustic wave can be reduced by setting θ between the Euler angles of the LiNbO3 substrate, a thickness of the silicon oxide film 104, and a thickness of the IDT electrode 103 to respective particular ranges.
Moreover, WO2008/044411 discloses a structure using LiTaO3 instead of LiNbO3.
In the boundary acoustic wave device described in WO2008/044411 1 and using LiTaO3, however, it has been discovered by the inventors of the present application that although leakage of the SH boundary wave to the silicon oxide film 104 is suppressed, energy of the boundary acoustic wave is leaked to the piezoelectric substrate made of LiTaO3 and a propagation loss cannot be sufficiently reduced.